Makatoxin I, a novel toxin isolated from the venom of the scorpion Buthus martensi Karsch, exhibits nitrergic actions

Buthus martensi Karsch venom exhibits nitrergic action in rat anococcygeus muscle (ACM), We have purified a novel toxin, makatoxin I (MkTx I), which exhibits nitrergic action, to homogeneity from this venom by a combination of gel-filtration, cation-exchange chromatography, and reverse-phase chromatography, Its purity was assessed by capillary electrophoresis and mass spectrometry, Its molecular weight was found to be 7031.71 +/- 2.88 as calculated from electrospray mass spectrographic data, The complete amino acid sequence was elucidated by sequencing of reduced and S-pyridylethylated toxin and a carboxyl-terminal peptide, P55-64, generated by the cleavage of toxin with endoproteinase Lys-C, The complete sequence of MkTx I is GRDAYIADSENCTYTCALNPYCNDLCTKNGAKSGYCQWAGRYGNACWCIDLPDKVPIRISGSCR. This toxin is composed of 64 amino acid residues and contains 8 half-cystine residues, Structurally, MkTx I has high similarity to Bot I and Bot II when compared with toxins from other scorpion species, The effects of MkTx I on nitrergic responses were investigated using the rat isolated ACM mounted in Krebs solution (37 degrees C, 5% CO2 in O-2), MkTx I (2 mu g/ml) markedly relaxed the carbachol-precontracted ACM; the relaxation was inhibited by the stereoselective inhibitor of nitric oxide synthase, N omega-nitro-L-arginine methyl ester (50 mu M). Thus, MkTx I is the first alpha-toxin that can mediate nitrergic responses in the rat isolated ACM.